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of water about 33 feet — the height of the column being less in proportion as the specific gravity of the fluid is greater - not so high if carried to the top of a mountain, and why? - temperature at which water boils varies with the height of the barometer -- boils at a less heat on the top of a mountain than at the bottom. The mode of ascertaining the height of mountains by means of the barometer. - Why this method is more to be relied on in tropical climates than in high latitudes, etc.
Pascal, in France, about the year 1647, was the first to make this experiment, which he did at the summit and foot of a mountain in Auvergne, called Le Puy-de-Dôme, the result of which led him to conclude that the air had weight. He also tried it at the top of several high towers, which convinced him of the weight of the atmosphere.
To register the daily altitudes of the barometer and the thermometer, would be a very useful exercise for the pupilteacher — and in its bearings branches out into a great many things.
The principle of the common pump might now be explained — how the atmospheric pressure which supports the mercury enables them to pump up water — having a model of a pump, or even with paper and pasteboard, showing the kind of tubes and nature of the valves, this may be clearly explained - pointing out how the valves act at each separate movement up and down of piston-rod -the limit to which water can be raised — the experiment of Torricelli, etc.
Supposing the atmospheric pressure about 15lbs. on the square inch—how much on five square inches? — how much on five inches square ? — on a square three inches on a side:
on the surface of the floor or the table ? — making them have recourse to the two-foot rule; pressure on the animal body, etc., and how counteracted. A fish under water has the pressure of the air, 15lb. on a square inch, besides the pressure from its depth in the water; — a basin of water with a live fish in it, when placed under the receiver of the air-pump and exhausted, the air-bladder expands, and the fish turns on its back. .
Children may easily be made to understand that the atmosphere is an aeriform fluid surrounding the globe, acted on like other bodies by the force of gravity, consisting principally of two airs or gases, varying in weight, and partly of a third, heavier than either of the others, but if placed upon each other in the order of their specific gravities, the heaviest nearest the surface of the earth, next heaviest in the middle, and the lightest at the top, that they would not remain in this order of superposition, as, for instance, the three fluids, quicksilver, water, and oil, would do; but the heavy one at the bottom would rise up and travel through the pores of the other, and the lighter one would descend, this being a property peculiar to bodies of this nature, and called the diffusion of gases. That, in addition to this, there is an atmosphere of vapour of water, arising from evaporation from the surface of the earth and of water, and which is in itself lighter than dry atmospheric air ; a cubic inch of water at the common atmospheric pressure forming about 1700 cubic inches of vapour; therefore a cubic inch of vapour of water is about too of the weight of a cubic inch of water — a cubic inch of common atmospheric air about goo.
Having called their attention to the fact that a substance lighter than water will, if plunged into it, rise to the top; that of two fluids the lighter will rest upon the heavier; arranging themselves according to their specific gravities as water upon mercury — oil upon water-cream upon milk -- they will easily understand why bodies lighter than air ascend in it, as the smoke from their chimneys — tell them to watch it, particularly on a still calm day — why it stands still and does not rise higher; the principle on which a balloon ascends, a soap-bubble, etc.
Again, why there is a draught up the chimney ; — the air rarefied, how this takes place; — why a current of air under the door and towards the fire — and another perhaps out of the room at the top of the door ?
The kind of resistance offered by the air to a falling body
this increases with the density — that, under the receiver of an air-pump, a guinea and a feather would fall at the same time.
As a simple experiment, showing the effect of rarefaction of air, the teacher might light a piece of paper, and while burning, place it in a tea-cup, and invert the cup in a saucer of water — the water will immediately be driven into the cup with a gurgling noise.
Again, in the practice which cooks have of putting an inverted tea-cup in a fruit pie, as they think with a view to prevent the syrup running over as the pie bakes, the air in the cup becomes rarefied, and is driven into the pie-dish, through the crust, into the atmosphere — when taken out of the oven it cools, the rarefied air in the cup is condensed, but as the mouth of the cup is surrounded with the juices of the pie, air cannot get into it, but it forces the liquid up.
The teacher explains why the resistance of the air in moving along is so little felt — some of the consequences of its being disturbed, and causes of its being put in motion — a breeze, a hurricane, etc. ; he would also speak of the forces of these at different velocities - the force varying as the square of the velocity. This short table might be the subject of a lesson :
It will be easy to calculate the force of the wind acting on a given surface, doing so in particular cases will be instructive.
Air as a vehicle of sound.
A bell under the receiver of an air-pump when exhausted, is not heard. . Bodies which produce the sensation of sound on the ear are in a state of vibration, as in a bell — the running a wet finger along the rim of a common drinking-glass, etc.
Here having to do with the instruction of children en
gaged in country occupations, I have called their attention in this, as in other subjects, to things coming under their observation, in a way something like the following:
Did you ever observe a woodman cutting down a tree at a distance; you could see the hatchet fall, and some time after that the sound of the blow came to your ear. Do you know the reason ?
Teacher. Light travels so fast that the time it is in coming from the hatchet to you is so small that it cannot be reckoned; so that when you see the hatchet fall, that is the instant the blow is given; but sound, coming at a very slow pace (1,142 feet in a second), takes as many seconds to get to your ear as when multiplied by 1,142, would give the number of feet between you and the man cutting down the tree.
For instance, if it were 2", his distance would be 1142 ft. x 2, if 3", 1142 x 3, and so on.
Did you ever see a man firing a gun at a distance, and, after seeing the flash, wonder why you did not hear the sound, or that you were kept considering how long it would be before the sound came ? Do you know the reason - can you explain it? Because sound lags behind, and the flash takes up no time in coming to the eye.
Supposing you were 5" before you heard the sound after seeing the fash, how far would you be off? -- 5x1143; 6", how far ? 6x 1142, and so on.
When we hear the Portsmouth guns here, if you could have seen the flash, do you think you could find out the distance betwixt this and Portsmouth ?
Supposing a man was standing where you could see him a mile off, and you saw the flash of his gun, how long would it be before you heard the sound ? A mile in feet divided by 1,142 would give the number of seconds before I could hear the sound.
Teacher. How do you think the sound gets to your ear? The air in the gunpowder suddenly expands and disturbs the air immediately about it, or the hatchet causes a vibration or tremulous motion in the wood, which sets the air in motion all round about; and this makes a sort of circular wave, beginning from a point which gradually enlarges,
one circle of the air of the atmosphere striking against another, until it reaches the ear, unless it meets with some hinderance in the way ; just as when you throw a stone into a smooth pond, a wave, beginning from the stone, spreads in every direction, until it reaches the bank. The air is as necessary to continue the sound up to your ear as the water is to make the wave come up to the bank.
Sound goes much quicker in water - nearly four times as quick as in air, and in solids from ten to twenty times quicker ; so that if you splash in the water at one end of a pond, the fish would hear you much sooner than a boy standing at the opposite side would do.
Now, in order that you may understand how well solids convey sounds, the next time you see a solid log of deal, or timber not very knotty and broken in the grain, at the carpenter's shop, set one of the boys to scratch at one end of it, and the rest of you go and listen at the other. Try the same on a block of stone, marble, etc.
But perhaps this will amuse you more : when you see the kettle on the fire, and you cannot tell whether it boils or not, place one end of the poker on the lid, the other to your ear, and it will tell you. If you strike with a hammer on a solid wall at one end, and some of you go and fix your ears against the other, you will most likely hear the sound of the blow twice—the first going along the wall you may call the wall-wave (coming more quickly), the second, a little after, through the air, coming with the airwave, we have talked of before. Try if you can hear two reports of the same knock by tapping with a hammer at the end of a log of wood-one along the wood, the other along the air. · You have heard of the wild natives of America-when they think their enemies are near, they lie down on the ground, and, by applying their ears to it, they can judge of the distance, and hear sooner than through the air.
Did you ever hear what is called an echo?
Supposing you were to clap your hands violently together, that creates a wave in the air which carries the sound along with it; now, if this wave happens to meet with a wall or a rock, or any obstable in its way, it is checked and